1. Field of the Invention
The present invention relates to a track servo control apparatus for an optical disk, and more specifically to an optical disk track servo control apparatus incorporated in an optical disk apparatus for a recordable optical disk, and for controlling a light spot to follow a track of the optical disk, by cooperation of a-lens actuator capable of slightly displacing the light spot in a direction traversing the track of the optical disk and a positioner capable of moving an optical head from one end to the other end of a radius of the optical disk.
2. Description of Related Art
In this technical field, JP-A-58-166541, JP-A-03-127337, JP-A-02-152035, JP-A-03-116544, JP-A-01-185843, JP-A-02-165431, JP-A-03-025728, JP-A-08-077589, JP-A-08-249678, and JP-A-05-040945 are known, the content of which are incorporated by reference in their entirety into this application (an English abstract of these Japanese patent application publications are available from the Japanese Patent Office and the content of these English abstracts are also incorporated by reference in their entirety into this application).
Referring to FIG. 1, there is shown a block diagram of one example of the prior art optical disk track servo control apparatus. A focusing lens 51 is located to oppose a face of an optical disk 10 having a spiral recording track, and projects light spot on the recording track and receives a light reflected from the optical disk. A lens actuator 52 rotates the focusing lens 51 along the optical disk face, for example, in the case of a rotating type, so as to slightly displace the focusing lens 51 and hence the light spot in a direction traversing the optical disk track. An optical head 53 is equipped with the focusing lens 51 and the lens actuator 52 mentioned above, records information into and reads out information from the track of the optical disk rotating at a substantially constant speed. This optical head 53 also detects a track error signal 20 (a position deviation signal indicating a deviation in position between the light spot and the track) on the basis of the reflected light returning to the lens 51 and in accordance with a conventional method known in the prior art. A positioner 54 moves the optical head 53 from one end to the other end of a radius of the optical disk, while maintaining the condition that the optical head 53 opposes to the optical disk face.
A servo signal processing circuit 55 receives the track error signal 20 from the optical head 53, and generates a corresponding servo control signal 20. A filter circuit 56 receives the servo control signal 20 from the servo signal processing circuit 55 and separates it into a high frequency component and a low frequency component divided with a predetermined frequency threshold value. On the basis of the high frequency component, the filter circuit 56 generates a control signal 24 for positioning the lens actuator 52 in an optimum position, and outputs the actuator control signal 24 to an actuator driving circuit 58. On the basis of the low frequency component, the filter circuit 56 generates another control signal 26 for positioning the positioner 54 in an optimum position, and outputs the positioner control signal 26 to a positioner driving circuit 57.
The positioner driving circuit 57 receives the positioner control signal 26 and supplies a driving current to a driving means (not shown) of the positioner 54. Here, the driving means of the positioner 54 is conventionally constituted of a voice coil motor (VCM). The actuator driving circuit 58 receives the actuator control signal 24 and supplies a driving current to a driving means (not shown) of the lens actuator 52.
Here, the frequency threshold value for separating the position deviation signal (track error signal) into the high frequency component and the low frequency component is fixed at a frequency which is conventionally set to be higher than a rotating frequency of a spindle of the optical disk. Therefore, as shown in FIG. 2A, in a low frequency band including the rotating frequency of the spindle of the optical disk, the track following control is mainly performed by the positioner 54, and in a high frequency band which does not include the rotating frequency of the spindle of the optical disk, the track following control is mainly performed by the lens actuator 52.
The driving means of the positioner 54 is conventionally constituted of the VCM as mentioned above. However, the VCM has various disadvantages. For example, the VCM needs large a magnet and coil, and is difficult to assemble. The efficiency of a magnetic circuit is low, and electric power consumption is large, so that heat is easily generated. Therefore, as the driving means for the positioner 54, it is desired to adopt a means for transferring the rotating force of a rotary motor by use of a gear mechanism.
However, since the gear mechanism inevitably involves friction, if the positioner 54 is frequently driven, its endurance becomes a problem. Therefore, it is desirable to lower the frequency threshold between the driving region of the positioner 54 and the driving region of the lens actuator 52 as shown in FIG. 2B in order to drop the frequency of the driving of the positioner 54 (the driving times of the positioner) thereby to ensure the endurance of the gear mechanism. However, if the frequency threshold is set lower than the rotating frequency of the spindle of the optical disk, the deviation in position caused by eccentricity of the optical disk must be followed by the lens actuator 52. In this case, from a condition shown in FIG. 3A to a condition shown in FIG. 3B, a deviation occurs in a positional relation between the reflected light from the optical disk and the focusing lens 51, so that there occurs various adverse influences in recording, including a change in the amount of light projected from the lens (since a light beam 51A has an intensity curve 51B having a peak on its center axis) and a deforming of the shape of the beam spot.